1. Field of the Invention
The present invention relates to a metal vapor discharge lamp, in particular, a metal vapor discharge lamp using an alumina ceramic discharge tube.
2. Description of the Prior Art
In recent years, in the field of metal halide lamps, it has been increasingly common that alumina ceramic is used as a material for a discharge tube in place of a conventional material of quartz glass. Since alumina ceramic is more excellent in heat-resistance than quartz glass, alumina ceramic is suitable for a discharge tube of a high pressure discharge lamp whose temperature becomes high during lighting. For this reason, a metal halide lamp using an alumina ceramic discharge tube can achieve high color rendering properties and high efficiency. Moreover, alumina ceramic has a lower reactivity with a metal halide that is sealed in the discharge tube than that of quartz glass, so that it is expected to contribute to further prolongation of the lifetime of the metal halide lamp.
For all the metal halide lamps using alumina ceramic discharge tubes that are commercially available at present, the limit of the electric power is 150W or less. In the future, when the lamp is used at a higher wattage, a problem may arise in the reliability of the sealing portion structure.
More specifically, the thermal expansion coefficient of tungsten or molybdenum that is used for a halide resistant portion of a feeding member inside a slender tube portion is significantly different from that of alumina. Therefore, in high-wattage lamps where the temperature of the discharge tube is further increased, cracks are generated in the sealing portion when the lamp is on, and leaks may occur in the discharge tube.
In order to achieve long life-time in the high-wattage lamps, use of a conductive cermet whose thermal expansion coefficient is substantially equal to that of alumina ceramic for the feeding member has been considered.
The electrodes of a lamps of this type are sealed, not by heating and pressing the side tube portions of the discharge tube, as in the case where quartz glass is used, but by melting a sealant such as frit glass and flowing the molten sealant therein. Therefore, in the portions that are not sealed with the sealant, a gap between the feeding member and the inner surface of the slender tube portion is generated (see JP-57-78763 A). Moreover, a high wattage lamp has a large discharge tube, and the larger the discharge tube is, the larger the gap becomes.
As described above, in the conventional metal halide lamp using alumina ceramic for the discharge tube, a gap is present between the feeding member and the inner surface of the slender tube portion. Therefore, when the lamp is turned on with the electrodes of the lamp being oriented in the vertical direction, luminous metal sealed inside the discharge tube tends to fall down into the gap between the feeding member and the inner surface of tile slender portion.
During the life of the lamp, when the luminous metal falls down into the gap, the metal contributes less to luminescence in the discharge space, so that sufficient vapor pressure cannot be obtained, and color temperature is changed significantly. In other words, even if the color temperature characteristics are sufficient immediately after the lamp turns on, the characteristics may be changed significantly, for example 100 hours after the lamp turns on. When the amount of the luminous metal sealed is increased in order to prevent this problem, the reaction between the luminous metal and the electrodes and the alumina is accelerated, so that the life-time characteristics deteriorate.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a metal vapor discharge lamp that has little color temperature change during continuous lighting for a long period and maintains stable characteristics by reducing the amount of the luminous metal that falls down into the slender tube portion.
In order to achieve the above object, a metal vapor discharge lamp of the present invention includes a discharge tube comprising a translucent ceramic discharge portion that defines a discharge space in which a luminous metal is sealed, slender tube portions provided on both ends of the discharge portion, a pair of electrodes provided with coils at the tips thereof, electrode supports that support the electrodes at one end and extend all the way to the ends of the slender tube portions on the side opposite to the discharge space at the other end thereof, and a sealant for sealing the ends of the slender tube portions on the side opposite to the discharge space so as to attach the electrode supports to the inner surfaces of the slender tube portions, wherein X greater than 0.0056P+0.394 is satisfied, where P is a lamp power (W) and X is a distance (mm) from the ends of the coils on the side of the slender tube portions to the ends of the slender tube portions on the side of the discharge space.
In this embodiment, the distance X from the tips of the electrodes including high-temperature positive columns and coils to the end of the slender tube portion on the side of the discharge space is set at a value that satisfies the above equation, so that the temperature in the vicinity of the end faces of the slender tube portions on the side of the discharge space can be kept at a temperature at which excessive luminous metal is liquid.
Thus, in the case where this metal vapor discharge lamp is turned on with the electrodes being oriented to the vertical direction, the amount of the luminous metal that falls down into the slender tube portion can be reduced from that in conventional lamps. As a result, the present invention can provide a metal vapor discharge lamp that keeps sufficient vapor pressure in the discharge space, allows little color temperature change in continuous lighting for a long period of time, and maintains stable characteristics.
In the above metal vapor discharge lamp, it is preferable that the sealant extends from the ends of the slender tube portions on the side opposite to the discharge space into the slender tube portions.
In this embodiment, the sealant is present inside the slender tube portions, so that the volume of the space in the slender tube portions is reduced, and therefore the amount of the luminous metal that falls down into the slender tube portion during lighting is reduced. Thus, this embodiment further suppresses the drop of the vapor pressure inside the discharge space. As a result, the present invention can provide a metal vapor discharge lamp that allows a further reduced color temperature change during continuous lighting for a long period of time, and maintains further stable characteristics.
In the above metal vapor discharge lamp, it is preferable that L less than Xxc3x9720.783Pxe2x88x920.0971 is satisfied, where L is a distance (mm) from the ends of the slender tube portions on the side of the discharge space to the ends of the sealant on the side of the discharge space.
In the above metal vapor discharge lamp, it is preferable that the slender tube portions are made of the same translucent ceramic as that for the discharge portion, and the electrode supports are made of a conductive cermet having a thermal expansion coefficient substantially equal to that of the translucent ceramic.
In this embodiment, cracks due to the difference in the thermal expansion coefficient hardly are generated during lighting, and leaks in the discharge tube can be prevented. Thus, the present invention can provide a metal vapor discharge lamp having a long lifetime, high color rendering and high efficiency.
As described above, the present invention provides a metal vapor discharge that has a reduced color temperature change during lighting and maintains stable characteristics.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.